Stent grafts are used to bridge a defect in the vasculature of a patient and can be deployed into the vasculature endovascularly. This requires that the device can be constrained into a small delivery device and be able to expand or be expanded when release within the vasculature.
Where there are side branches to the vasculature it may be necessary to provide an aperture in the stent graft, known as a fenestration, to enable access from a deployed stent graft to that side branch. Such a fenestration may be reinforced with a peripheral circular ring stitched to the graft material around the fenestration. It is also desirable in some situations to provide a side branch stent graft extending through the fenestration and into the side branch.
PCT Publication WO 2005/034808 entitled “Fenestrated Stent Graft” describes the use of resilient reinforcing rings in stent grafts and the teachings therein are incorporated herein in their entirety.
To obtain a good seal of the branch stent graft within the fenestration an inflatable balloon can be used to expand the branch stent graft into the fenestration and for this purpose the reinforcing ring must be able to resist expansion of its diameter. At the same time the ring must be resilient so that it can be distorted into its deployment configuration but when released expand back to its circular configuration. In this specification the term resilient when used in relation to a wire used to manufacture a reinforcing ring refers to a wire which is substantially inextensible but which has a spring function so that when distorted and released returns to its original configuration.
This invention will be discussed in relation to the application of a reinforcing ring to a fenestration but such a ring may have greater applicability.
Generally such a reinforcing rings is manufactured from a metal known as a superelastic metal such as, but not restricted, to a nickel titanium alloy known as Nitinol. To form a ring of a superelastic metal the desired final shape is formed from a wire on a former and then the wire on the former is heated above a temperature which sets the wire in the new shape. Upon cooling the ring holds it formed shape and can be distorted and resiliently returns to the formed shape.
The reinforcing rings discussed in PCT Publication WO 2005/034808 mentioned above when formed from a resilient wire each have substantially circular loops at the terminal ends of the wire. These loops prevent the sharp end of the wire puncturing the vasculature into which the stent graft is deployed. When it is desired to electropolish rings incorporating these prior art loops it is necessary to straighten out the ring but the prior art loops do not permit efficient electropolishing because parts of the loops touch the wire of the ring.